#include<stdio.h>

char f(int start, int len)
{
	int b, i, n;
	char a[3]={'a', 'b', 'c'};
	n= sizeof(a)/sizeof(char);
	b=start+len;
	if(b>n)
	   printf("NULL");
	else if(start>=1)
	  {
	   len=len+start;
	    for(i=start;i<len;i++)
	      printf("'%c' ", a[i]);
	  }
	else
    {
    	for(i=start;i<len;i++)
    	printf("'%c' ", a[i]);
    }
}

main()
{
	int s, l,n;
	printf("Nhap so phan tu can bat dau: ");
	scanf("%d", &s);
	printf("Nhap so phan tu muon lay: ");
	scanf("%d", &l);
	f(s,l);
    getch();
}
